Turbofan engines generally include a fan and a core arranged in flow communication with one another. A first portion of air over the fan may flow past the core through a bypass airflow (defined between the core and an outer nacelle) and a second portion of air over the fan may be provided to the core.
The core of the turbofan engine generally includes, in serial flow order, a compression section, a combustion section, a turbine section, and an exhaust section. In operation, the air provided to the core flows through the compression section where one or more axial compressors progressively compress the air until it reaches the combustion section. Fuel is mixed with the compressed air and burned within the combustion section to provide combustion gases. The combustion gases are routed from the combustion section to the turbine section. The flow of combustion gasses through the turbine section drives the turbine section and is then routed through the exhaust section, e.g., to atmosphere.
The fan of the turbofan engine defines a fan pressure ratio. Certain turbofan engines are designed with a fan defining a relatively low fan pressure ratio, such as a fan pressure ratio less than about 1.4. However, such fans may experience an aeromechanical flutter when operated at such low fan pressure ratios. In order to alleviate such flutter, turbofan engines typically include a variable area fan nozzle to unload the fan, or more particularly, a plurality of fan blades of the fan. Typically, the variable area fan nozzles include a nacelle having an aft end capable of expanding in diameter. However, such variable area fan nozzles may be relatively complicated and heavy, potentially reducing an overall efficiency of the turbofan engine and increasing its expense.
Accordingly, other exemplary turbofan engines include a variable pitch fan to alleviate the aeromechanical flutter. Again, however, variable pitch fans can also be relatively complicated. Accordingly, a turbofan engine having a fan defining a relatively low fan pressure ratio and including one or more components capable of alleviating aeromechanical flutter would be particularly beneficial. More particularly, a turbofan engine having a fan defining a relatively low fan pressure ratio and including one or more components capable of alleviating aeromechanical flutter without requiring relatively heavy additional systems would be particularly useful.